Substantially transparent films are employed in numerous different industries to provide a protective and/or decorative coating to a surface, such as with floor and countertop coverings and automotive coatings. Such films can serve to protect the coated substrate from damage due to the environmental stress, radiation, temperature, scratching, and chipping. An example of an abrasion-resistant floor covering is sold by Perstop Flooring AB (Sweden) under the trademark PERGO.
Other attempts to improve the hardness of films prepared from cross-linkable resins, while maintaining optical transparency, have incorporated inorganic particles with diameters varying from the nanometer range to the micron range and with varying solid forms. However, the nanometer range particles used in previous attempts (for example, colloidal silica) typically have not been crystalline in nature and have not been in the form of nanocrystalline particles. As a result, such particles do not impart the same degree of improvement in the physical properties of the cross-linked resin film when employed at comparable concentrations as nanocrystalline particles. Further, micron range particles do not impart the same degree of clarity in the cross-linked resin film as nanocrystalline particles when employed at comparable concentrations, due to a greater extent of light reflection caused by the larger micron range particles.
The degree of dispersion of inorganic particles throughout a cross-linked resin film affects the maintenance of optical transparency. Inefficient particle dispersion results in agglomeration, leading to higher transmitted and reflected haze and lower transparency in films containing the particles. Higher dispersion can be achieved through treatment of the surface of the particles in order to provide compatibility between the particles and the environment or film matrix in which they are dispersed. Different surface treatments have been employed for use with dispersed inorganic particles, resulting in different film/particle compositions.
U.S. Pat. No. 5,853,809 discloses a clearcoat coating composition that includes reactive, colorless, inorganic microparticles dispersed in a coating composition. These inorganic microparticles may have an average diameter in the range of from about 1.0 to about 1000 nanometers, preferably from about 2 to about 200 nanometers, and most preferably from about 4 to about 50 nanometers. The coating composition includes a binder system having a cross-linkable resin for forming a transparent film, and a polyvalent linking agent, such as a silane coupling agent, intended to bind the inorganic microparticles to the resin. Colloidal silicas are preferred as the microparticles, and such silicas may be in colloidal, fumed, or amorphous form. The patent does not disclose the use of nanocrystalline particles, or any particles comprising alumina, titania, ceria or zinc oxide, which differ substantially from silica. The patent does not disclose that nanocrystalline particles may be surface treated before being included in a cross-linkable resin or that such treated nanocrystalline particles have advantages in clearcoat coating compositions.
U.S. Pat. No. 5,993,967 describes a coated ceramic powder comprising ceramic particles and a siloxane star-graft polymer coating and a method for preparing same. U.S. Pat. No. 6,033,781 describes the use of a coating composition and preparation method disclosed in U.S. Pat. No. 5,993,967 for application to a plurality of ceramic particles. U.S. Pat. No. 5,993,967 and U.S. Pat. No. 6,033,781 are each incorporated herein by reference.
U.S. Pat. No. 6,001,163 describes transparent coatings from aqueous-organic solvent mixtures containing an epoxy functional silane, a tetra-functional silane, and a multifunctional compound from the group of carboxylic acids and anhydrides. Although the patent describes a siloxane composition that can be applied to substrates to form a transparent coating, the siloxanes are disclosed as film-forming compositions rather than particle coatings or modifiers. The patent does not describe the use of nanocrystalline particles or the surface-treatment of such particles.
Inorganic and organic particles can be modified by absorption, ion exchange, and covalent bonding. Surface modification by absorption and ion exchange require the particle surface to have appropriate chemical characteristics. Reactions that enable covalent bonding to inorganic particles generally involve reactions on hydroxylic surfaces.
Inorganic particles may be coated by graft polymerization and encapsulation. Inorganic powders may be coated by the precipitation of powders in the presence of suspended powders or by spray drying of polymer solutions containing the powder. However, these conventional methods yield uneven coatings and the formation of coated agglomerates.
Transparent films that exhibit an improved hardness, scratch resistance, abrasion resistance, and optical transparency are desired.